Germ cell

A germ cell is any biological cell that gives rise to gametes of a sexually-reproducing organism. They create sperm in men and eggs and women, which are used to reproduce.

Background
Hugo de Vries decided to revisit the concept of inheritance after his rediscovery of Gregor Mendel's works, but nobody knew what the units of inheritance were at the time. Additionally, nobody knew anything about the physical location of these units of inheritance; the use of microscopes did not occur until the 1890s and 1900s. At this time, scientists began to develop the cell theory of development, believing that embryos developed as the result of cell multiplication. German scientists believed that cells made tissues, tissues made organs, organs made organ systems, organ systems made an organism, and organisms made up a population.

In the body, there are two major kinds of cells: germ cells and somatic cells. Almost all kinds of cells in the body are somatic, having a diploid number of chromosomes (46 chromosomes, half of which came from the mother, the other half of which came from the mother). These cells under go mitosis, in which the 46 chromosomes duplicate all of their genetic material. In meiosis, diploid cells produce four cells with half of the chromosome number, a process which occurs in women's ovaries and men's testes. This process is also how germ cells produce egg cells in a woman and sperm cells in a man. There are major differences in how the cells are made; men have seminiferous tubules, while the cells which produce sperm are on the outside of the tube, and the sperm go towards the lumen and collect and mature there. Men produce sperm until the time they die. In women, germ cells are born with the number of eggs they will have with their lives, and, when they reach the point of menstruation, they have a maturation process, in which one egg eventually develops. Over time, women's eggs are frozen, and, as they get older, some eggs can have mistakes in the process of meiosis, leading to chromosomal problems in developing fetuses. As women get older, their chances of getting birth defects increases.

Description
Around the 1900s, Theodor Boveri and Walter Sutton developed a theory of inheritance, studying sea urchins and grasshoppers. They studied chromosome morphology and they discovered that the movement of chromosomes as the germ cells were made through the process of meiosis. Sea urchins have separate male and female sexes, and, while the distinguishing features are not visible externally, it is easy to remove the gonad and study development of the eggs or sperm - fertilization is external, so it was also easy to study this event and the development of the embryo. Boveri discovered that cancer was probably caused by cells with abnormal chromosomes in them, and that these abnormal cells would create tumors; this was not confirmed until the 1960s and 1970s. The duo discovered that germ cells work in pairs - a pair of chromosomes, male and female. This was in accordance with the segregation of alleles in Mendel's studies, and they realized that the invisible units of inheritance were located on the chromosome. They developed the "Chromosome Theory of Inheritance" out of these ideas.

In sperm, the chromosomes are located in the head, and the tail (whose mitochondria makes it move) falls off when it contacts the egg. The sperm carries all of the DNA and chromosomes, and karyotype photos can show the chromosome count; sperm should produce 23, but sperm which produce abnormal zygotes can produce 22 or less, or even fragments of sperm, causing spontaneous miscarriages. When males and females undergo meiosis, there are often abnormal germ cells (eggs and sperm), which produce abnormal zygotes; most of the times, these do not plant, but, when they do stick, they can create miscarriages or children with birth defects.